Does a solar cover keep pool warm or make pool warm?

No not really but the more you cover the more heating there will be. The cover is designed to heat the water under the cover so its effect will be diminished by the lack of total pool coverage. You will notice some warming just the same.

You should try to cover your entire pool, but if there's a gap here and there it won't matter. A good Solar cover raises your pool temp. about 10-15 degrees.

After use, allow it dry and roll it up nicely.